Finding resonant freq of a antenna using S11

As in my previous post i am trying to identify a big batch of unknown freq mobile & portable antennas. I have been trying to find the bands they were designed for by using 8720 analyzer set to S11 return loss and then switch to smith chart to check for 50 ohms. Problem is that they are not coming up close to the frequency i think they should be, even a known factory antenna also comes up eitherhigher or lower that it should be. Am i missing something here?

One of the most common mistakes is to think that the antenna at resonance is 50 ohms. It may not be (depends entirely upon the feedpoint impedance design). Also, if the antenna is tuned to 50 ohms, and your analyzer is not calibrated with a good 50 ohm load, then your apparent smith chart center will be the impedance of the calibration load, not 50 ohms.

Remember, near resonance, the impedance changes almost linearly with frequency, from below 50 ohms to above 50 ohms, so what appears as a shift in frequency may really be a shift in impedance. Finally the ground plane has a really big effect on impedance, so you might consider how you are supplying ground plane. I wonder if you might want to do a separate S21 between two similar antennas to see where they have maximum gain.